The proton exchange membrane belongs to the solid-state electrolyte. Although it is different from the aqueous electrolyte in the voltaic cell, it also has functions similar to the electrolyte solution which can pass positive and negative ions, as it is a conductor. The main function of the proton exchange membrane is to transmit protons, such polymers in the fuel cell are the most important elements, and directly affect the performance and life of the fuel cell. The requirements of the film are as follows: (a) a good proton transmission capacity (high ion exchange capacity, a uniform microphase separation); (2) able to be isolated from the fuel (gas or methanol) contacts; (3) the catalyst layer in close contact with the film (a good adhesion); and (4) have sufficient mechanical strength and heat tolerance.
The current manufacturing method of the proton exchange membrane can be classified into four types, as shown in FIGS. 1A to 1D, which are schematic diagrams for showing the possible structures of the current sulfonated polymer used in a proton exchange membrane, in which the hatched segments represent the hydrophobic segments, and the unmarked segment represents the hydrophilic segments. The sulfonated polymer shown in FIG. 1A has a linear long chain structure. Although it can be attached with a plurality of sulfonate groups, the sulfonate groups are not enough. The sulfonated polymer shown in FIG. 1B has a branched main chain structure. Although it can be attached with a plurality of sulfonate groups, the proton conductivity thereof is low at high temperatures. The sulfonated polymer shown in FIG. 1C has a linear long chain structure. Although it has more sulfonate groups and higher IEC, the mechanical properties and thermal stability are on the low side. The sulfonated polymer shown in FIG. 1D has a linear main chain structure and has a partially dense distribution of the sulfonic acid groups. Therefore, it has relatively better physical and chemical properties required for a proton exchange membrane.
FIG. 1D shows the locally and densely sulfonated polymer developed in the past two years. This method can gather the sulfonate groups more efficiently to form a locally hydrophilic segment. Therefore, the membrane will have a more concentrated sulfonate hydrophilic end and a longer hydrophobic end within an appropriate IEC value, which has the same conductivity as Nafion and keeps excellent mechanical properties. However, in addition to the conductivity, a good proton exchange membrane must have the advantages of good dimensional stability, thermal stability, high proton transition capability, chemical stability, process stability, and low cost for production. In the aspect of dimensional stability, the proton exchange membrane formed by the abovementioned locally and densely sulfonated polymer still has higher water absorption because the sulfonated positions and the number of sulfonate groups cannot be precisely controlled, and thus the length swelling and the thickness swelling of the proton exchange membrane are also affected.
It is therefore necessary to provide a polymer of fluorine-containing sulfonated poly(arylene ether)s and a method of manufacturing the polymer capable of being used for producing a proton exchange membrane having a better dimensional stability, in order to solve the problems existing in the conventional technology as described above.